In the art of down hole percussion drilling, such as for mining, quarrying, water well drilling, etc., the drilling elements comprising a drilling tool are frequently coupled together by threaded screw structures. In these coupled drilling elements, the thread design is critical since failure often occurs in the screw structure. Whenever failure in the screw structure does occur, the initial crack always starts at the thread root. This is due to the high stress concentrations located at the root of the threads when the screw structure is subject to severe loading.
In prior art screw structures, the root portion has a curvature defined by a portion of a circle, tangently adjoining the two flanks of the adjacent threads. Generally, the larger the radius of the circle defining the thread root, the lower the stress concentration. However, when the size of the circle surpasses a maximum allowable radius, the stress concentrations at the end of the arc, where the thread flanks tangently adjoin the root, become very high and thus provide the starting point for crack propagation. Alterenatively, when the size of the circle defining the root curvature decreased below a minimum allowable radius, the stress concentrations become very high at the bottom of the root portion. Thus, circular root curvatures are confined to radii sizes falling between maximum and minimum limits, depending on such factors as thread pitch and the like. Therefore, there is need for a root curvature having a very large radius, yet relieving the high stress concentration located where the thread flanks tangently adjoin the root portion.
The U.S. Pat. No. 4,295,751 to Holmberg, issued Oct. 20, 1981, discloses an example of a screw structure for percussion drilling elements. As shown in FIG. 3, the screw structure includes a root portion 22, 22' defined by a portion of a circle having a radius R. The Holmberg screw structure is deficient in that the circular root portion 22, 22' is subject to the above described limitations preventing the circular root curvature from surpassing the maximum allowable radius, and thus restricting the relief of additional stress concentration.
The U.S. Pat. No. 4,549,754 to Saunders et al, issued Oct. 29, 1985, discloses a screw structure for drilling elements including a root portion 20 defined by two tangential circles having proximate centers for their respective radii 31, 34. The two radii 31, 34 scribe respective circles, of which a portion of each define the root portion 20. The two radii 31, 34 are used so that a circular root having a larger equivalent can be provided while maintaining the same thread-to-thread contact length between mating male and female screw structures. However, the enlarged curvature at the root portion 20 creates the unacceptable effect of reducing the wall thickness at the bottom of the thread root 20. A reduction in the wall thickness is not tolerable for screw structures used in the percussion drilling art because the wall thickness comprises the smallest supporting area for the threads, and hence a propensity for failure exists in this area. Additionally, the stress concentration at the intersection of the non-load bearing flank 35 and the root portion 20 is very large, and is therefore another unacceptable effect produced by the root shape of the Saunders screw structure.
Accordingly, the root portions shown in the prior art are deficient in that they rely on a circular root curvature. The circular root portions are shown to be inadequate since an increase in the radius of the circular portion defining the thread root will unacceptably decrease the supporting wall thickness at the root portions. Or, if the wall thickness is not decreased, an increased circular radius defining the root portion will increase the stress concentration at the end of the arc where the thread flanks adjoin the root, and thus provide the starting point for crack propagation.